1. Field of the Invention:
This invention relates to a process for the recovery in the form of tetravalent fluoride of uranium from an aqueous phosphate-bearing solution.
2. Discussion of the Background:
Phosphate-bearing solutions and in particular phosphoric acid which are produced by sulphuric attack of phosphate-bearing ores contain an amount of uranium which is generally from 0.040 to 0.200 g/l, depending on the origin of the ore.
Because of the high level of interest in uranium in the nuclear industry many processes have been developed for recovering that metal from such solutions.
Some such processes consist of extracting uranium in hexavalent form with organophosphorus solvents such as mixtures of di-2-ethylhexylphosphoric acid and trioctylphosphine oxide. This solvent mixture is commonly referred to as di-2-EHPA+TOPO and described for example in U.S. Pat. No. 3,711,951.
Other processes are based on extracting uranium in the tetravalent form by using other organophosphorus solvents. For example, mixtures of mono and dioctylphenylphosphoric acids are used in a process referred to as the O.P.A.P. process. Alkylpyrophosphoric acids such as octylpyrophosphoric acid are used in a process referred to as the O.P.P.A. process. The O.P.A.P. processes is disclosed in U.S. Pat. No. 3,835,214 and Belgian Pat. No. 870,346. The O.P.P.A. process is disclosed in U.S. Pat. No. 2,866,680.
In the case of processes in which uranium is recovered in tetravalent form, the uranium-charged organophosphorus solvent is generally subjected to a reextraction operation using an aqueous solution of hydrofluoric acid. This causes the uranium in the solution to become a suspension of grains of fluoride which can be recovered from the suspension after separation of the solvent.
Thus, in the case which involves using a solvent such as octylpyrophosphoric acid, the uranium contained in the solvent is re-extracted by a solution of hydrofluoric acid in a proportion of 15% by weight to provide a suspension of UF.sub.4.2.5H.sub.2 O. However, that fluoride species often occurs in the form of very fine needles which form a thixotropic emulsion with the acid mother liquors and the solvent. The components of this thioxtropic emulsion are very difficult to separate by settlement and filtration so that it is frequently necessary to recover the uranium by centrifugation. Centrifuging an emulsion of two liquid phases and a solid phase in a relatively highly concentrated hydrofluoric medium is a problem which is virtually impossible to solve with the equipment available at the present time. And even if centrifugation is possible, there remains in the solvent a substantial amount of fluoride. What is finally recovered is a fluoride which still contains a substantial proportion of mother liquors and which has to be subjected to a drying operation. Those disadvantages detrimentally affect the price of recovered uranium.
There thus remains a strongly felt need for a facile process which will permit the effective recovery of uranium.